While I was trying to update my system with "apt-get update" I always got the error as shown below :

E: Encountered a section with no Package: header
E: Problem with MergeList /var/lib/apt/lists/us.archive.ubuntu.com_ubuntu_dists_natty_main_binary-i386_Packages
E: The package lists or status file could not be parsed or opened.
E: _cache->open() failed, please report.

and I also couldn’t even start the Synaptic Packet Manager,
I did some research in internet and found the solution. The solution was to remove all the package source listing and then rebuild it.

First delete all of the previously available listings

sudo rm /var/lib/apt/lists/* -vf

And then use apt-get to rebuild your package listings

sudo apt-get update

The rebuilding takes a bit time, so you will have to be patient.

References :

  1. http://ubuntuforums.org/showthread.php?t=1750755#7
  2. https://answers.launchpad.net/ubuntu/+source/update-manager/+question/155062